In the learning theory, the vast majority of both theoretical and applied research have concentrated on the developed world. In contrast, developing, in particular, emerging economies have drawn much less attention. Moreover, empirical outcomes are conflicting, with some studies revealing learning-by-doing to have a positive impact, but others finding learning-by-doing to have a negative effect on the performance. Therefore, this study is conducted through the Metropolis-Hastings and Gibbs samplers in the context of a Cobb-Douglas specification to evaluate the effects of learning-by-doing on firm performance on a panel data of the 227 manufacturing firms listed on the Vietnamese stock market. A Bayesian mixed-effects regression used allows for capturing the varying effects of all the researched firms. The study found that firm-specific learning-by-doing has a strong positive influence on firm performance. This finding is accordant with the predictions of the learning theory, many previous investigations, as well as the fact that in a fast-growing economy like Vietnam, firm-specific learning-by-doing is closely associated with economic growth. Some helpful policy implications proposed are aimed at increasing productivity for firms in emerging economies.